1. Field of the Invention
The present invention relates to an optical fiber network, and more particularly, to a gain flattened multi-stage optical amplifier system for an optical fiber network.
2. Discussion of the Related Art
In a long distance optical fiber network, optical power signals generally decrease with distance so that it is usually necessary to use in-line amplifiers to boost the optical signal power levels. To provide in-line amplification, erbium-doped fiber amplifiers (EDFAs) are most commonly employed. In a multichannel optical system, gain flattened amplifiers are desired so that each channel maintains substantially the same optical power and optical signal to noise ratio. Furthermore, flat gain characteristics together with constant output power are desired despite variable span losses. That is, the amplified signal power levels should be constant even when signal losses vary over the distances between adjacent amplifiers.
EDFAs typically have flat gain from about 1547 nm to 1559 nm when pumped to achieve an average inversion level of 70%. Flat gain operation from 1525 to 1565 nm can be achieved by adding gain flattening filters. However, the flat gain characteristics degrade with input power variations as the number of WDM channels and/or span losses change.
S. Y. Park et al. in xe2x80x9cAccurate Control of Output Power Level in Gain-Flattened EDFA with Low Noise Figurexe2x80x9d Proceedings of the 11th Conference on Integrated Optics and Optical Fibre Communicationsxe2x80x9423rd European Conference on Optical Communications (IOOC-ECOC ""97), No. 448, Vol. 3, pp. 43-46 (1997) describe a system that addresses these problems as shown in FIG. 5 of this specification. As shown in FIG. 5, the system of Park et al. uses two stage EDFA having EDFA optical amplifiers EDF1 and EDF2. However, to achieve a constant total gain, the amplification of each EDFA optical amplifier EDF1 and EDF2 is maintained by controlling the associated pump laser power. That is, the amount of laser pump power for each stage is increased or decreased as the total input power is increased or decreased. To accomplish this, the input signal power to each section is tapped using signal taps T to be monitored by the associated control part C. Then, the control part C, having a pump laser, provides pumping power to the associated EDFA optical amplifier section through the couplers W in accordance with the monitored signal powers. In addition, FIG. 5 shows filter F which removes undesired wavelengths and isolators I that eliminate backward traveling light. Also, the Optical Supervisory Channel (OSC) power is measured using wavelength selective coupler W2 and controller C2. The received OSC power level is used to adjust the voltage controlled attenuator VCA to maintain constant per channel output power levels, for a wide range of span loss variations.
The system of Park et al. suffers from a number of limitations and drawbacks. For example, this pump control scheme has limited dynamic range due to unstable operation at low pump powers. In addition, the control parts C must actively adjust the laser pumping power. As a result, the electronics that provide monitoring, feedback and control are both cumbersome and expensive. Furthermore, successful constant gain of the twostage optical amplifier of FIG. 5 relies upon the continuous, reliable operation of the control parts C.
Accordingly, the present invention is directed to an optical amplifier system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the invention is to provide a multi-stage optical amplifier system that has flat gain and constant output per channel in a multichannel optical fiber network.
Another object of the present invention is to provide a two-stage optical amplifier system that is inexpensive and uncomplicated to manufacture and operate.
Another object of the present invention is to provide a two-stage optical amplifier system that provides constant gain for each section without the need for active control.
Another object of the present invention is to provide an optical amplifier system that compensates for variable span losses.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an optical network includes a transmission line; a first wavelength division multiplexer for receiving N optical signals each at a different wavelength and for multiplexing together the N optical signals, and for outputting the multiplexed signal to the transmission line; at least one optical amplifier system disposed in the transmission line, the optical amplifier system including a first stage optical feedback loop amplifier defining a first optical feedback wavelength and a second stage optical feedback loop amplifier defining a second optical feedback wavelength; and a second wavelength division multiplexer for receiving and demultiplexing the multiplexed signal from the transmission line.
In another aspect, a two-stage optical amplifier system for amplifying optical signals between first and second portions of a transmission line includes a first optical amplifier having an input coupled with the first portion of the transmission line and an output; a first feedback loop coupled with the input and the output of the first optical amplifier, the first feedback loop defining a first optical feedback wavelength; a second optical amplifier having an input coupled with the output of the first optical amplifier and an output coupled with the second portion of the transmission line; and a second feedback loop coupled with the input and the output of the first optical amplifier, the second feedback loop defining a second optical feedback wavelength.
In another aspect, an optical network includes a transmission line having first and second ends; a transmitting terminal for outputting light to the first end of the transmission line, the light having N wavelengths each corresponding to wavelengths of the multichannel signal and one wavelength different than the N wavelengths, the N +1 wavelengths of light having a predetermined total power; and a line amplifier for receiving light from the second end of the transmission line, detecting the total power of the received light, amplifying the received light to a power level equal to the predetermined total power, and retransmitting the amplified received light.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.